Impact of the bromination of carbazole-based D–π–A organic dyes on their optical and electrochemical properties and visible-light-driven hydrogen evolution

Brominated dyes, 2C-n (n = 1–5), 3C-4 and 4C-4, were prepared through bromination of three carbazole-based D–π–A dyes, 2C, 3C and 4C with N-bromosuccinimide (NBS). The detailed structures of the brominated dyes were confirmed by 1H NMR spectroscopy and mass spectrometry (MS). The introduction of the Br atom on the 1,8-positon of carbazole moieties led to blueshifted UV-vis and photoluminescence (PL) spectra, increased initial oxidation potentials and enlarged dihedral angles, indicating bromination enhanced non-planarity of the dye molecules. In the hydrogen production experiments, with the increase of the Br content in brominated dyes, the photocatalytic activity increased continuously (except 2C-1). The dye-sensitized Pt/TiO2, 2C-4@T, 3C-4@T and 4C-4@T, exhibited high hydrogen production efficiencies of 655.4, 877.9 and 905.6 μmol h−1 g−1, respectively, which were 4–6-fold higher than those of 2C@T, 3C@T and 4C@T. The enhanced performance of photocatalytic hydrogen evolution was attributed to decreased dye aggregation resulting from the highly non-planar molecular structures of the brominated dyes.


Introduction
In recent years, solar energy has been regarded as a substitute for traditional fossil energies, such as petrol and gas. The utilization of solar energy to produce clean energy, hydrogen, is one of many important concerns in the eld of renewable energy nowadays. 1,2 Photocatalytic hydrogen evolution from water splitting using TiO 2 as the catalyst has been proposed for nearly 50 years. 3,4 In this process, the hydrogen evolution follows a mechanism where the excited electrons from TiO 2 induced by light are transferred to Pt, and reduce water to hydrogen. However, because the energy gap of TiO 2 is very wide (ca. 3.2 eV for anatase), it can only absorb ultraviolet light (ca. 4% to 6% in the solar spectrum). Many studies are devoted toward improving the light-harvesting ability of the catalyst systems. [5][6][7][8] Among them, dyes are used for anchoring on TiO 2 to absorb visible light, which is called dye sensitization. 9 In addition to good light-harvesting capability, the dye-TiO 2 system can effectively separate excited electrons and holes to avoid charge recombination. The main types of dyes include ruthenium complexes, [10][11][12] porphyrins, 13 xanthenes 14,15 and metal-free organic compounds. Due to their designability and accessibility, metal-free organic compounds have attracted great attention in recent decades. [16][17][18] A metal-free organic compound usually has a D-A or D-p-A conjugated structure. The electronic push-pull effect between the donor (D) and acceptor (A) in the structure is not only conducive to broadening the light absorption range, but also benecial to electron-hole separation and charge transfer. When designing the structure of D-p-A conjugated dyes for photocatalytic hydrogen evolution, researchers mainly focus on three aspects: (i) donors, which are moieties including triphenylamine, [19][20][21] phenothiazine, 22,23 coumarin 24 and carbazole 25,26 with strong electron-donating properties and the ability to adjust the energy level and broaden the light absorption range; (ii) p link (spacer) [27][28][29][30] which can accelerate electron transfer from D to A; (iii) side chains, [31][32][33][34] which can control the combination mode of dyes in the dye-TiO 2 system by adjusting hydrophilicity and steric hindrance.
Metal-free organic compounds are also used in dyesensitized solar cells (DSSCs), which have almost been replaced by perovskite solar cells now. In the research on DSSCs, dye aggregation is considered one important factor, which usually greatly affects the photoelectric performances of DSSCs. The typical method to inhibit dye aggregation is to add an anti-aggregation agent, chenodeoxycholic acid (CDCA), into the electrolyte. [35][36][37] By contrast, strategies to prevent dye molecules from aggregation by intrinsic molecular structure design are favoured by scientists, including (i) introducing alkyl chains [37][38][39][40] and (ii) incorporating a non-planar bulky aromatic skeleton into dye molecules. [41][42][43][44][45][46] The photovoltaic performance of DSSC was signicantly improved by the inhibition of dye aggregation due to steric hindrance of alkyl chains and nonplanar structures. Regarding dye-sensitized photocatalytic hydrogen evolution, inhibition of dye aggregation has also been proved to greatly improve hydrogen production efficiency. 47 In our previous works, we reported three carbazole-based D-p-A dyes, 2C, 3C and 4C, and their application for DSSC 48 and photocatalytic hydrogen evolution. 49 In those systems, benetting from large dihedral angles between carbazole moieties in dye molecules, dye aggregation was decreased and relatively good photocatalytic performance was afforded. Herein, we report a strategy for further change of the planarity of dye molecules by introducing Br into the 1 or 8 positions of carbazole moieties through bromination with NBS, as shown in Scheme 1. The introduction of Br can change the dihedral angles, thus increasing the non-planarity and effectively preventing dye aggregation. The enhanced non-planarity of dyes should also have great inuence on their optical and electrochemical properties, and more importantly, their hydrogen production performance.

Materials
Three multi-carbazole-based aldehydes (2C-CHO, 3C-CHO, and 4C-CHO) were synthesized mainly through Suzuki coupling and the Knoevenagel reaction from carbazole. The three acids (2C, 3C and 4C) were prepared from the aldehydes as described in our previous work. 48 NBS, cyanoacetic acid and piperidine were used as received. Platinized TiO 2 nanoparticles (0.5 wt% Pt/TiO 2 ) were prepared using a photoreduction method reported in our previous work. 50 The sacricial electron donor was aqueous triethanolamine (TEOA, 10 vol%) neutralized with HCl aqueous solution.

Synthetic procedures
Synthesis of brominated aldehyde compounds. 2C-CHO (0.306 g, 0.5 mmol) and a predetermined amount of NBS were added to DMF (30 mL) and cooled using an ice bath The reaction mixtures were stirred at ambient temperature for 24 hours in the dark. Aer extraction with dichloromethane, the organic layers were washed with water, dried over anhydrous sodium sulphate and concentrated under reduced pressure. The residues were puried by column chromatography (petroleum ether : CHCl 3 : ethyl acetate = 10 : 4 : 1 in volume). The brominated products from 2C-CHO were named 2C-CHO-n, where n denotes the sample with different feeding molar ratios (n (NBS) : n (2C-CHO) = 1-5).
Synthesis of brominated acid compounds. The brominated aldehyde compounds (100 mg) and cyanoacetic acid (100 mg, about 10 eq.) were dissolved in CHCl 3 /acetonitrile (10 mL/10 mL), and then piperidine (0.5 mL) was dropped in. The reaction solutions were stirred at reux temperature for 12 hours. Aer cooling, dilute hydrochloric acid was added for neutralization. The organic layers were separated, dried with anhydrous sodium sulphate, and concentrated under reduced pressure. The residues were separated by column chromatography (THF : MeOH = 10 : 1 in volume) to produce red powders. The obtained brominated acid compounds were named 2C-n, 3C-4 and 4C-4 except for 2C-1, the brominated acid compounds obtained were all mixtures, and the yield was 60-80%.

Measurements
1 H NMR spectra were recorded on a 500 MHz Bruker Avance III HD spectrometer using CDCl 3 or DMSO-d6 as the solvent. FT-IR spectra of the samples were measured on an IRPrestige-21 instrument (Shimadzu, Japan) by a transmission method using the KBr pellet technique. Matrix assisted laser desorption ionization time-of-ight mass spectrometry (MALDI-TOF MS) was performed on a Bruker autoex III (Bruker, Germany). Cyclic voltammetry (CV) measurements were performed with a CHI650E electrochemical workstation. The redox potentials of the dyes were measured in a DMF solution (3 × 10 −4 M) containing 0.1 M tetrabutylammonium hexauorophosphate (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV s −1 , using AgCl/Ag and platinum wire as reference and counter electrodes, respectively. All potentials were calibrated with ferrocene/ferrocenium (Fc/Fc + ) as an internal standard. UV-vis absorption spectra were collected using a UV-2550 spectrophotometer (Shimadzu, Japan). Photoluminescence (PL) and time-resolved uorescence decay curves of the dyes in DMF solution (1 × 10 −5 M) were obtained on a spectrophotometer of U-3310 (Hitachi, Japan).
H 2 evolution measurement. In a Pyrex bottle, Pt/TiO 2 (0.100 g) was dispersed in DMF (9.8 mL) and stirred under ultrasound for 20 min. Then, the organic dye solution (0.2 mL, 2 × 10 −3 mol L −1 in DMF) was added and stirred for another 10 min to allow the organic dyes to adsorb on the catalyst. Neutralized TEOA (90 mL, 10 vol%) was added into the Pyrex bottle, and the suspension was stirred in an ultrasonic bath for 5 min and purged with N 2 for another 30 min to remove O 2 . The Pyrex bottle containing the suspension was placed under the irradiation of a 250 W halogen lamp. The amount of evolved hydrogen was quantied using a gas chromatograph (TCD, 13X molecular sieve column) with N 2 as the carrier gas and normalized to the 1 g of the photocatalyst.

Synthesis and structures of brominated dyes
All brominated dyes were prepared by bromination and Knoevenagel reactions, as shown in Scheme 2. In the bromination reaction, the aldehyde compounds, 2C-CHO, 3C-CHO, and 4C-CHO were reacted with NBS. The position and number of Br in the molecule greatly depended on the amount of NBS. When NBS was 1 eq. to 2C-CHO, the reaction was easy to perform, and the yield was high due to the high activity and selectivity of the 6-position of carbazole moiety in 2C-CHO. When the dosage of NBS was further increased, the substitution has to take place at the 1 and 8 positions. Because of large steric hindrances, further bromination at 1 and 8 positions was difficult and uncontrollable. In addition, unfavorable oxidation of the thiophene group in 2C-CHO by NBS took place, resulting in a lower number of Br in the products than the theoretical values. The oxidation became more intense as the dosage of NBS increased. When the amount of NBS was greater than 5 eq. to 2C-CHO, the yield of the brominated product was extremely low. Therefore, 2C-CHOn (n = 1-5), 3C-CHO-4 and 4C-CHO-4 were prepared. Then, Knoevenagel reactions between the brominated aldehyde compounds and cyanoacetic acid were carried out and gave the corresponding brominated acid compounds, 2C-n (n = 1-5), 3C-4, and 4C-4.
The structures of brominated aldehydes were determined by 1 H NMR spectroscopy. It can be seen from the 1 H NMR spectrum of 2C-CHO-1 in Fig. 1a . 1b). This change can also be found when comparing 1 H NMR spectra of N-butylcarbazole with its mono-, di-, tri-and tetra-Br-substituted compounds (Fig. S1 †). Therefore, it can be concluded that as the amount of NBS increases, the number of Br on the aldehyde compounds continues to increase despite somehow deviating from theoretical calculation due to oxidation. The resolution of 1 H NMR spectra of brominated acid compounds is poor (see Fig. 1c), so mass spectrometry was used for more accurate analysis. As shown in Fig. 1d, the molecular peaks of 2C-1 are at 758 corresponding to one-Br-substituted compound (1Br); 2C-2 has two series of peaks, one at 758 corresponding to 1Br and another at 836 corresponding to two-Brsubstituted compound (2Br); 2C-3 mainly contains peaks at 836 for 2Br and at 914 corresponding to three-Br-substituted compound (3Br); 2C-4 mainly contains peaks at 914 for 3Br; 2C-5 mainly contains peaks at 914 corresponding to 3Br and at 994 corresponding to four-Br-substituted compound (4Br). Therefore, the compositions of 2C-n can be inferred, as shown in Table 1. Also, from 1 H NMR spectra and mass spectrometry (see Fig. S2 and S3 †), 3C-4 and 4C-4 are found to mainly contain 3Br and 4Br, as shown in Table 1. It must be noted, however, that the molecular formula given in Table 1 is presumably obtained from mass spectrometry. Except for 2C-1, the substitution position of Br is not necessarily as shown in Table 1. Without selectivity in the bromination reactions, the brominated acid compounds except 2C-1 might contain a variety of homologues with different Br substitution positions. However, considering the steric hindrance and electronic effect, the molecular structure of each component  of brominated acid compounds is described with the molecular formula shown in Table 1 for convenience.

Optical properties
The UV-vis absorption spectra of 2C-4C and all brominated dyes in DMF solution are shown in Fig. 2a and b. In Fig. 2a, compared with 2C, the peak at 350-500 nm of 2C-1 attributed to intramolecular charge transfer (ICT) shows a slight redshi, which is due to the increased conjugation degree by 6-position substitution of electron-donating Br. 2C-n (n > 1) showed a different trend from 2C-1. With the increase of n (i.e., the increase of Br content), the peak at 350-500 nm was continuously shied toward a low wavelength. It was also found that the peak intensity at 300-325 nm (attributed to p-p* transition of conjugated bicarbazole) kept declining while the peak intensity at 250-285 nm (attributed to p-p* transition of single carbazole) continued to increase. It is believed that the heavier bromination occurred at the 1 and 8 positions, the larger steric hindrance by Br existed, which led to increasing dihedral angles between heterocycles and a reduced conjugation degree. That is to say, bromination breaks down the conjugation, making the conjugated bicarbazole tend to be a single carbazole. As shown in Fig. 2b, the UV-vis spectra of 3C-4 and 4C-4 also have similar characteristics to that of 2C-4, with a blueshi compared with that of 3C and 4C.
The photoluminescence (PL) spectra of the dyes are shown in Fig. 2c and d. Consistent with the results of UV-vis spectra, the    PL peak of 2C-1 has a small red shi relative to 2C, while 2C-n (n > 1) all show a blueshi. The wavelength of the maximum peak decreases with the increase of n, which also evince that the conjugation degree of dye molecules decreases in response to bromination. In addition to that, for samples testing at the same concentration, when n rises from 1 to 3, the PL intensity increases sharply. This result is reasonable because of the electron donation of Br and decreased dye aggregation caused by the lowered molecular planarity. When n continues to increase from 3 to 5, the uorescence intensity decreases, which might be a result of the lowered conjugation degree of the heavily brominated dyes. Fig. 2d shows the change of PL spectra of 2C-4-4C-4, which have an obvious redshi compared with their non-brominated counterparts, 2C-4C.

Electrochemical properties
The electrochemical behaviors of the dyes were measured by cyclic voltammetry (CV), and the results are shown in Fig. 3.
Although most brominated dyes are mixtures, it can be seen from Fig. 3a that the initial oxidation potentials of dye 2C-n increase constantly from 0.7 V to more than 1.0 V with the increase of n. From Fig. 3b, 2C-4C displayed much lower initial oxidation potentials than 2C-4-4C-4. The signicant impact of the introduction of Br on the electrochemistry of dyes is the result of the decreased conjugation degree aer bromination. On the other side, the redox process of dyes is reversible based on the symmetrical curve shape, indicating that the dyes have decent stability in the redox process.

Theoretical calculation
Density functional theory (DFT) calculations at the B3LYP/6-31G* level were carried out to further understand the geometrical congurations and electron distributions of 2C-nBr and tetra-brominated derivates, 3C-4Br and 4C-4Br are shown in Table 1.
For 2C and 2C-nBr containing two carbazole moieties, with the increase of n, the dihedral angle between two carbazole increases from 37.3 to 38.4°and the dihedral angle between carbazole and thiophene increased from 18.3 to 21.4°(see Fig. S4 †). The increase is not very large because Br has a small atomic size, but may greatly destroy the conjugation degree of the molecule, considering the great changes in optical and electrochemical properties. 3C-4Br and 4C-4Br also exhibit larger dihedral angles than 3C and 4C, respectively.
From Table S1, † the HOMO and LUMO levels gradually enhance with the increase of n, just like the CV results. E 0-0increases monotonically, indicating that the introduction of Br leads to a narrow absorption range, which is also consistent with the UV-vis data of 2C-n (except 2C-1). In addition, the optimized structures and electric distribution in the HOMO and LUMO levels of 2C-4Br-4C-4Br are presented in Table 3. The HOMO is mainly distributed on electron-donating carbazole moieties, while the LUMO is predominantly delocalized over the cyanoacrylic acid segment and extended to thiophene moiety. This kind of electronic distribution is favorable to the effective separation of charge holes during photocatalytic hydrogen evolution.

Photocatalytic hydrogen production
The photocatalytic systems, Pt/TiO 2 sensitized with each dye were named dye@T, for example, 3C-4@T. Without the presence of dyes or light irradiation, the hydrogen production efficiency was lower than 5 mmol h −1 g −1 , demonstrating the importance of the sensitization and light absorption of these dyes.
The results of photocatalytic hydrogen evolution over 2C@T and 2C-n@T with 10 vol% neutralized TEOA as the sacricial agent under visible light irradiation (l $ 420 nm) are shown in Fig. 4a. In the rst hour of illumination, the hydrogen production efficiency over 2C@T was 99.2 mmol h −1 g −1 , while that over 2C-1@T increased to 321.5 mmol h −1 g −1 , which is due to the wider visible light absorption range brought by Br. The hydrogen production efficiency of 2C-2@T was slightly lower than that of 2C-1@T, which is due to the narrower visible light  absorption range. With the further increase of n, the hydrogen production efficiency over 2C-n@T continuously rises, from 461.3 mmol h −1 g −1 for 2C-3@T to 655.4 mmol h −1 g −1 for 2C-4@T and 618.6 mmol h −1 g −1 for 2C-5@T. Then, the hydrogen production performances of Eosin Y (EY), 2C-4C and their brominated products with 4 eq. NBS were compared together, and the results are shown in Fig. 4b. The hydrogen production efficiency over EY@T was only 122.6 mmol h −1 g −1 and those over 2C@T, 3C@T, and 4C@T were 99.2, 224.6 and 202.6 mmol h −1 g −1 , respectively. The brominated dyes, 4@T, 3C-4@T, and 4C-4@T had hydrogen production performance of 655.4, 877.9, and 905.6 mmol h −1 g −1 , respectively, 4-6 folds higher activity than their counterparts, 2C@T, 3C@T, and 4C@T. The apparent quantum yields (AQY) of the selected photocatalytic system 3C-4@T were investigated and the data are listed in Table S2. † Under monochromatic light irradiation by band-pass lters, l = 450, 475, 500 nm, the AQY values of 3C-4@T are 2.55%, 1.79%, and 1.64%, respectively. The photocatalytic activity can also be discerned from transient photocurrent responses, as shown in Fig. S5. † The photocurrents of 2C-4/TiO 2 , 3C-4/TiO 2, and 4C-4/TiO 2 are much higher than that of nonbrominated dyes.
Therefore, it can be concluded that the introduction of Br into dye molecules is very effective for improving photocatalytic hydrogen evolution. Based on the above-mentioned data from UV-vis, PL, CV testing, and DFT calculations, bromination could give rise to two results, blueshi absorption spectra and enhanced non-planarity of dye molecules (excluding 2C-1). Since a narrow absorption range is harmful for utilizing visible light, non-planarity is supposed to have a close correlation with the improvement of hydrogen production performance. Enhanced non-planarity due to the introduction of Br means low dye aggregation, which is considered the reason for the strong recombination of photoinduced electron-hole pairs. When each dye molecule is not aggregated with other dye molecules and independently adsorbed on Pt/TiO 2 , as shown in Fig. 5, the energy stored in excited dye molecules aer absorbing light should not easily be consumed through PL emission, and the photogenerated electron-hole can be effectively separated and transferred to Pt/TiO 2 .
Transient uorescence can obtain the uorescence lifetime (s), which can be used to analyze the electron-hole separation. The decay curves and t of all brominated dyes are shown in Fig. 6a-c. Brominated dyes have increased t compared with non-brominated dyes. Lower dye aggregation by bromination makes it difficult for excited brominated dyes to lose energy through intermolecular relaxation and thus obtain large s, which can give enough time for electrons to migrate to Pt/ TiO 2 . 51 On the other hand, the decay curves and t of 3C@T and 3C-4@T are shown in Fig. 6d. Because of the PL quenching caused by the electron transfer from the dye to Pt/TiO 2 , the t of dye-sensitized Pt/TiO 2 has inverse meaning compared with that of pure dye. Unlike the small difference between the t of 3C and 3C@T, 3C-4@T has a t of 0.90 ns, much lower than the 1.66 ns of 3C-4. This indicates that the electron transfer from 3C-4 to Pt/TiO 2 is very fast and correspondingly charge recombination is reduced. 52,53 The stability of photocatalytic hydrogen evolution for 3C-4@T was investigated. As shown in Fig. 7a, with the increase of irradiation time, the activity of hydrogen production decreased signicantly. By comparing the absorption spectra of 3C-4 before and aer hydrogen production (see Fig. 7b), it was found that the peak at 350-500 nm attributed to ICT disappeared aer irradiation due to the instability of the cyanoacrylic acid segment. 49,54 The degradation of cyanoacrylic acid segments under light can be conrmed from the decreased absorption peak at 2210 cm −1 (ascribed to CN groups) in Fig. 7c. In addition, according to the study of EY, there may also be debromination in the presence of Pt and hydrogen. Because of the debromination, the disappearance of the peaks at 9.3 ppm and the change of the peaks at 4.4-4.8 ppm was found in Fig. 7d aer light irradiation.   This indicates that the molecular structure of the brominated dyes needs further improvement. Therefore, we have now started to use arylboronic esters to react with Br on brominated dye molecules. Furthermore, vulnerable cyanoacrylic acid segments will be replaced with carboxyl acid segments. By strengthening the non-planarity with starburst aryl groups and using durable stable carboxyl acid segments, new 1,8-arylsubstituted dyes should obtain highly stable and effective hydrogen production activity.

Conclusions
In summary, a series of brominated carbazole-based D-p-A organic dyes were synthesized and served as sensitizers for photocatalytic hydrogen evolution. With the increase of bromination degree, the brominated dyes showed a blueshi of UV-vis absorption and PL spectra (except 2C-1), and an increase of HOMO and LUMO levels, indicating that the substitution at the 1-and 8-positions by Br decreased the molecular planarity. The brominated dyes exhibited much higher activity than the original 2C, 3C and 4C. The dye-sensitized Pt/TiO 2 photocatalytic systems, 2C-4@T, 3C-4@T and 4C-4@T, displayed great hydrogen production efficiencies of 655.4, 877.9 and 905.6 mmol h −1 g −1 , respectively, a 4-6-fold enhancement compared to those of photocatalytic systems based on corresponding non-brominated dyes. The inhibition of dye aggregation, which is due to the low molecular planarity caused by the introduction of Br, leads to the higher photocatalytic efficiency. Although these brominated dyes have poor stability, our ndings can provide useful ideas for dye design for photocatalytic hydrogen evolution.

Conflicts of interest
The authors declare no competing nancial interest.